Geoscience Reference
In-Depth Information
in separation for transition from liquid water to water vapor is much larger, the
latent heat of vaporization
for liquid water is also greater. The amount of energy
needed also depends on the temperature at which the phase changes occur.
Molecules in warm water are already a little farther apart than in cold water, so
rather less energy is needed and the latent heat of vaporization is slightly lower at
higher temperatures. For this reason,
, the latent heat of vaporization of water,
is temperature dependent and when the temperature,
T
C
, is given in
l
°
C,
l
is
calculated by:
l
=
2.501
−
0.002361
T
C
MJ kg
−
1
(2.1)
Atmospheric water vapor content
As discussed in Chapter 1, the atmosphere is a complex mixture of gases with
nitrogen and oxygen being the dominant constituents. However, in the tropo-
sphere, water vapor is a particularly important constituent, with a variable con-
centration that is typically a few percent. When describing the concentration of
water vapor in the air, it is convenient to speak in terms of air comprising just
two constituents, i.e., water vapor and 'dry air.' Dry air is the general description
of all the other gases present. The sum of these two constituents is then referred
to as 'moist air.'
If the densities of water vapor, dry air, and moist air in an air sample are
r
v
,
r
d
and
r
a
, respectively, the proportion of water vapor in the moist air can be charac-
terized in several different ways. One is as the
mixing ratio, r
, which is defined as
the ratio of the mass of water vapor to the mass of dry air in the moist air sample,
expressed in terms of densities as:
=
r
r
r
v
(2.2)
d
Another common way to define the water vapor content of an air sample is as the
ratio of the mass of water vapor to the mass of moist air. This ratio,
q
, is called the
specific humidity
of the air, expressed in terms of densities as:
r r
r rr
q
==
+
v
v
(2.3)
(
)
a
v
d
Because
r
a
, the numerical values of mix-
ing ratio and specific humidity are usually very similar. Strictly speaking, both
r
and
q
are dimensionless quantities, but often
q
is referred to as having units of
kg kg
−1
or, more often, gm kg
−1
. The specific humidity of air in the troposphere is
generally in the range 0-30 gm kg
−1
.
r
v
is always considerably less than
r
d
and
